Arsenical bronze

Arsenical bronze is an alloy in which arsenic is added to copper as opposed to, or in addition to tin or other constituent metals, to make bronze. The use of arsenic with copper, either as the secondary constituent or with another component such as tin, results in a stronger final product and better casting behaviour.

Since copper ore is often naturally contaminated with arsenic, the term "arsenical bronze" when used in archaeology is typically only applied to alloys with an arsenic content higher than 1% by weight, in order to distinguish it from potentially accidental additions of arsenic.

Although arsenical bronze occurs in the archaeological record across the globe, the earliest artifacts so far known have been found on the Iranian plateau in the 5th millennium BCE. Arsenic is present in a number of copper containing ores (see table at right, adapted from Lechtman & Klein, 1999) and therefore some contamination of the copper with arsenic would be unavoidable. However it is still not entirely clear to what extent arsenic was deliberately added to copper and how much its use arose simply from its presence in copper ores that were then treated by smelting to produce the metal.

A possible sequence of events in prehistory involves considering the structure of copper ore deposits, which are mostly sulphides. The surface minerals would contain some native copper and oxidised minerals, but much of the copper and other minerals would have been washed further into the ore body forming a secondary enrichment zone. This includes many minerals such as tennantite, with their arsenic, copper and iron. So the surface deposits would be used first, and with some work deeper sulphidic ores would have been uncovered and worked, and it would have been discovered that the material from this level had better properties.

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